"Random spots" sounds like a sign of hot tub rash which is caused by bacteria due insufficient sanitizer levels.
Can you post your latest water test results, how old the water is and if you've cleaned the tub recently? If you are running a bromine tub then what are you using as an oxidizer - chlorine, ozone, non-chlorine shock (MPS)? Bacterial colonies can form in the hot tub piping system and can be hard to kill if they form biofilms on the pipes. Depending on your answers, you may have to run a cleaning cycle on the tub.
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